Contact blocks have a protruding operating member such as a shaft (reciprocating or rotary) and are specifically built for panel mounting or for DIN rail mounting, but not for both. A contact block built for panel mounting typically is secured against the back of a panel, the operator shaft protruding to the front of the panel through a hole. The close proximity of the panel necessitates rear-facing terminal screws for making the wiring connections. A contact block built for DIN rail mounting typically has a rear-facing slot that embraces a DIN rail. Front-facing terminal screws are provided because the DIN rail and/or its supporting structure would block access to rear-facing terminal screws.